Virtual FileSystem API

[Gozala]

From Decentralized Web Meet-Up feedback it seems that most people would really want FileSystem API compatible #8 API using IDBMutableFile so I think we should just build one and put it on npm.

[Gozala]

I've implemented random-access-storage from Dat ecosystem in separate repo since IDBMutableFile is not WebExtensions specific API it's better in user-space.

I'm happy to help porting underlying storage API for IPFS and SSB on top of IDBMutableFile, but that should happen outside of this repo. We could possibly use https://github.com/libdweb org if desired.

@lidel @soapdog @dominictarr please let me know if that is something you need help with and where you think that effort should take place.